Fossil fuels represent the main source of energy for the past century and this situation is likely to continue in the coming decades. The demand for fossil fuels today exceeds 5000 gigatons of carbon per year and creates major concerns for nature's carbon balance and which instigate or impact on global climate change. While it is important to develop “carbon-neutral” energy to cope with total energy consumption, advancement in technologies related to carbon capture and sequestration (CCS) are equally important in addressing the issue of global climate change.
The process of “amine scrubbing” is one of the most cost effective methods currently available to remove carbon dioxide from flue gasses such as those emitted by coal fired power stations. In amine scrubbing, an aqueous solution of alkanolamine, such as a 30 wt % monoethanolamine (MEA), is used to absorb carbon dioxide via chemisorptive formation of a carbamate, carbonate or bicarbonate species in the presence of water. Regeneration of the amine usually requires the application of elevated temperatures (100° C. to 150° C.) to cleave the covalent bonds between the amine group and the carbon dioxide to thereby release the gaseous carbon dioxide and regenerate the amine. The bonding energies of these covalent bonds are typically about 100 kJ/mol and hence the regeneration of the amine expends significant energy (also termed as the “energy penalty”) in the process of absorbing carbon dioxide gas. Accordingly, the disadvantages of using liquid amine scrubbing include the corrosive and volatile nature of amine and the aforementioned high energy penalty. Furthermore, a liquid amine scrubber requires periodic top-up of the amine solvent due to loss of the liquid amine during the gas-scrubbing and regeneration process, e.g., evaporative losses, leakage, etc. This in turn translates to increased operating costs attributed to the need to periodically replenish the liquid amine.
Accordingly, solid sorbents have been proposed as alternatives to conventional liquid amine scrubbers. In this regard, certain solid sorbents have been found to be effective for carbon dioxide separation and capture. In particular, a number of porous systems have been investigated for carbon dioxide capture and numerous functionalized amines or nitrogen-rich solid materials have been reported to demonstrate enhanced carbon dioxide uptake property. Known solid sorbents include activated carbon, zeolites, amine-modified silicas, hybrid crystalline solids (such as Metal Organic Frameworks (MOP), Zeolitic Imidazole Frameworks (ZIP)) and porous polymers (such as Covalent Organic Frameworks (COF), Conjugated Microporous Polymers (CMP) and Hyper Cross-linked Polymers (HCP)). Furthermore, solid sorbents comprising melamine functionality obtained by immobilization of melamine on the surface of porous silica or incorporation of melamine into a COF structure have also been proposed in the art. However, these mesoporous solid sorbents typically require complex synthesis procedures and involve high production costs, thereby limiting their applications in large scale industrial use.
Poly-melamine formaldehyde (PMF) is a known co-polymer material that has been widely used as in plastic laminate and overlay materials, thermal insulators, sensors and lenses. Additionally, porous PMF has been considered for use in the absorption of compounds such as benzene and water. However, known PMF resins and foams produced by conventional methods, e.g., use of soft templates or by sol-gel processes, typically possess surface areas of less than 100 m2/g. While PMF synthesis using colloidal silica templates have managed to exhibit higher surface areas of 220 m2/g, the total surface area is still insufficient for performing effective gas adsorption. As a result, these porous PMF materials remain unsatisfactory for fabricating materials for adsorbing gases such as carbon dioxide. Furthermore, synthesis methods involving the use of colloidal silica templates are also considerably more complex and lack scale-up potential.
One study disclosed a porous melamine-based polymer formed from a reaction between melamine and aryl-aldehydes. This porous melamine-based polymer is reported to exhibit high surface areas of about 1,000 m2/g. In one embodiment, this porous melamine-based polymer is formed from a reaction between melamine and a combination of di- and tri-phenyl-aldehydes. One drawback of this melamine-aldehyde polymer is that, due to the presence of the large aryl groups in the polymer structure, large and irregular interstitial spaces may occur within the polymer structure that are thought to increase the pore size and reducing the total surface area. Furthermore, the presence of large aryl groups decreases the density of the amine functional groups available for reaction with gas molecules. This in turn may have an adverse effect on the effectiveness of the polymer if used for gas absorption. Another drawback of the above disclosed porous melamine-based polymer is the relatively high cost associated with procuring the aryl-aldehydes reactants.
In another known method of making a melamine-based polymer, it has been proposed to form a PMF polymer aerogel by reacting melamine with formaldehyde in the presence of a strong sodium hydroxide base catalyst in an aqueous medium. However, the synthesis method involves a multiple-step process, including at least one pH modifying step and a curing step. The sodium hydroxide base catalyst also needs to be neutralized by a pH modifier such as an acid. The sodium hydroxide catalyst in the aqueous medium results in hydroxide functionalization of the bridging groups between the melamine groups of the produced cross-linked PMF. The hydroxide functionalization renders the PMF unsuitable as a gas adsorbent for gases such as carbon dioxide. Furthermore, due to the multiple steps in the process and the need to neutralize the sodium hydroxide base, the process is not suitable or is at least inhibited from being used on an industrial scale to produce porous PMF.
Accordingly, there is a need to provide a novel solid sorbent material for use as a gas adsorber which overcomes, or at least ameliorates one of the disadvantages mentioned above. In particular, there is a need to provide an inexpensive solid sorbent for use as a gas adsorber that is durable, exhibits high surface area for adsorption, is relatively easy to synthesize and regenerate. There is further a need to provide a method for producing this solid sorbent.